"Discomfort and persistent pain in the low back can be caused by a shortened psoas muscle. The psoas (actually 2 large muscles that attach the trunk to the legs) connects to both sides of the lumbar spine. It's main muscular duty is to flex the hip (bring the thigh toward the abdomen). When the hip is taken into extension (think of your back leg when you walk), a psoas that is engaged and won't release can cause the lumbar spine to arch and shorten causing pain in the low back (hyperlordosis). The pain can also be in the front of the hip crease or even at the sits bone. Typically, the psoas is only tight on one side leading to further imbalance and discomfort."